1. Field of the Invention
This invention pertains generally to power amplifiers, and more particularly to a digital polar and ZVS contour-based hybrid power amplifier.
2. Description of Related Art
Linear power amplifiers (PAs) such as classes A, B, and AB are biased for peak output power and consequently suffer from poor efficiency at backed-off power levels. Techniques such as transformer-combiner based PA and envelope tracking have improved the efficiency of linear PAs. However, transformer-combiner based PAs are limited by die size constraints whereas envelope tracking suffers from supply regulator bandwidth and efficiency problems. The recent interest in digital PAs has produced numerous schemes, such as the digital envelope modulator and switching mixer PA. However, each of these new offerings while improving aspects, including PA linearity, similarly suffer from efficiency degradation.
Architectures based on switching PAs, such as supply modulation (e.g., polar and polar loop), theoretically offer high efficiency even at very low output power levels. However, they suffer from supply regulator inefficiency, which is particularly pronounced in response to handling wide bandwidth envelope variations. Duty cycle modulation and dynamic load modulation for class-E (switching) PAs, is directed at high peak efficiency, while it provides poor efficiency at low output powers. At low output power levels, the class-E PA operates sub-optimally as the so-called zero voltage switching (ZVS) conditions, which otherwise guarantee maximum efficiency circuit operation, are not satisfied resulting in significant losses and poor efficiency. The recent ZVS contour-based PAs achieve high efficiency under back-off; however, they have a dynamic range which is limited by the achievable duty cycle modulation.
Accordingly, a need exists for a new PA architecture which is readily implemented and capable of providing high efficiency even at significantly backed-off power levels. The present invention fulfills that need as well as others, while overcoming the shortcomings of previous PA approaches.